In oil and gas well drilling operations, multiple wells may be drilled in a single field. Because a particular field may have reasonably similar rock formations at other locations of the field, information and knowledge acquired from the drilling of one wellbore in the field may be used to improve drilling operations of subsequent wellbores in the same field. Some of the information and knowledge that could be used in subsequent wellbore drillings can be acquired through analysis of one or more already drilled wellbores. Improvements in drilling operations of subsequent wellbores may result in significant economic benefit, for example, by extending longevity of drilling tools (e.g., drill bits) and by reducing the amount of time that would otherwise take to drill the wellbores.
To illustrate, information about a rock formation associated with an already drilled wellbore may be useful in properly selecting drilling parameters, such as weight-on-bit (WOB) and bit rotational speed (RPM), in drilling subsequent wellbores in the same field as the already drilled wellbore. Further, information and knowledge acquired during the drilling of a wellbore may be used in selecting appropriate drill bits for drilling subsequent wells in the same filed. The selections of appropriate WOB, RPM and drilling bit(s) are significant factors in optimizing drilling operations and reducing drilling costs.
Some existing methods of analyzing drilling operations rely on well logs and/or drilling data associated with one or more already drilled wellbores to improve drilling programs for drilling subsequent wellbores. For example, the method described in U.S. Pat. No. 4,914,591 relies on WOB, RPM, rate of penetration (ROP), drilling fluid related parameters as well as a numerical model of a drill bit to determine the compressive strength of a rock formation. As well known to those of ordinary skill in the art, compressive strength (e.g., unconfined compressive strength (UCS)) of a rock formation is highly valuable information in the efficient drilling of wellbores. For example, the type of drill bit, WOB, and RPM selections for drilling wellbores can be appropriately selected based on UCS information acquired from one or more previously drilled wellbores.
In some methods of analyzing drilling operations, UCS and related information are determined based on a sonic log, a gamma ray log, and at least one other well log, such as a neutron density log. While sonic logs and gamma ray logs are commonly available following a drilling of a wellbore, neutron density logs and other well logs are often unavailable, or when available, they are available for a small segment of a wellbore that is inadequate to enable a satisfactory analysis of a drilling operation. Further, many methods of analyzing drilling operations often require determining porosity of a rock formation to be able to determine the UCS of the rock formation. However, because porosity may not correlate with UCS for all rock types, additional steps may be required to perform a satisfactory analysis of the drilling operation.
Accordingly, having the capability to analyze a drilling operation based on commonly available well log data and without the need for infrequently available log data is desirable. For example, methods and systems of determining UCS and other wellbore information (e.g., abrasiveness of a rock formation and total work done by a bit) based on a sonic log and a gamma ray log and without the need for a neutron density log can be highly beneficial in cost effectively analyzing drilling operations and in optimizing subsequent drilling operations.
The drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments.